A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, the patterned substrate and/or the reticle may be inspected for, e.g., process control and verification. There are various techniques for performing such inspection, including the use of scanning electron microscopes, and various specialized inspection systems, which may be used to detect defects on the reticle and/or measure, for example, critical dimension (CD) of the patterns on the substrate, overlay error between successive layers formed on the substrate. One type of specialized inspection system is a scatterometer in which a radiation beam is directed onto a target of the pattern on the surface of the substrate and one or more properties of the scattered or reflected radiation beam, for example, intensity at a single angle of reflection as a function of wavelength, intensity at one or more wavelengths as a function of reflected angle, or polarization as a function of reflected angle are measured to obtain a spectrum from which a property of interest of the target may be determined. Determination of the property of interest may be performed by various techniques, such as but not limited to reconstruction of the target structure by iterative approaches (e.g., rigorous coupled wave analysis or finite element methods), library searches, and/or principal component analysis. Two main types of scatterometer are known. Spectroscopic scatterometer that directs a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation beam scattered into a particular narrow angular range. Angularly resolved scatterometer that use a monochromatic radiation beam and measure the intensity of the scattered radiation beam as a function of angle.
Objective lens systems are used in these scatterometers for directing and/or focusing the radiation beam onto the object of inspection (e.g., the reticle, the target of the pattern on the surface of the substrate) and for collecting and/or imaging the scattered or reflected light from the object of inspection. The amount of information obtained from the collected light and/or from the images of the object may depend on the numerical aperture (NA) of the objective lens system and the wavelengths of the radiation beam used in the scatterometers. The higher the NA of the objective lens system and the wider the spectral band of wavelengths used in the scatterometers, the greater is the amount of information that can be obtained from the illuminated object of inspection. However, the highest NA and the maximum spectral bandwidth that can be used in an inspection system are limited by the design and configuration of one or more lenses in the objective lens system.
There are three types of high NA objective lens systems currently used for scatterometry applications: refractive, reflective, and catadioptric. Certain disadvantages are associated with the use of these current objective lens systems. One of the disadvantages of current high NA refractive objective lens system is that the working distance is relatively small. For example, the working distance is generally less than 0.35 mm for high NA (e.g., 0.9-0.95). Another one of the disadvantages is that the spectral band of wavelengths over which the current high NA refractive objective lens system can operate without compromising optical performance is limited to wavelengths ranging from about 450-700 nm. Use of current refractive objective lens systems outside this spectral band of wavelengths (e.g., below 450 nm wavelength, above 700 nm wavelength, between 410-450 nm wavelengths, between 700-900 nm wavelengths, at deep ultra violet (DUV) wavelengths, at infrared (IR) wavelengths) results in a loss of resolution due to chromatic aberrations (axial color aberrations). Loss of resolution can lead to reduced accuracy of the scatterometer measurements.
One of the disadvantages of current catadioptric and/or reflective objective lens systems is that they have a large Petzval sum that is far from zero (i.e., they do not have a flat field curvature) and as a result induce field curvature aberration. Pupil aberration is another one of the disadvantages of the current catadioptric and/or reflective objective lens systems due to their large field curvature and pupil size. Further, the current catadioptric and/or reflective objective lens systems suffer from obscuration that reduces the amount of collected light, and hence, the amount of information that can be collected from the object of inspection.